Exact Measurements - or resize to specific dimension
First point can't currently be snapped from the start.
It is a known limitation that is currently being looked at, it is part of a broader limitation that includes among other things not being able to set a base point for moving before transforming
Current recommended workaround is to eyeball the first point, precisely snap the second one, then return to the first one and move it while using snapping.

Edit
As of commit Gizmo Library: New Snap Gizmo from 27 May 2020, there is now a new generic special type of gizmo whose purpose among other things is providing snapping before use for various tools and operators in Blender, like base point snapping for transforms such as move or rotate, or snapping the first measure point of a ruler without having to backtrack